## Monday, July 06, 2009

### Wrocław summary

So, I did not get around to live blog from the XXVth Max Born meeting "The Planck Scale". The main reason was, that there were no hot news or controversial things presented, rather people from the different camps talked about findings that a daily reader of hep-th had in some form or the other already noticed. I don't want to create the impression that it was boring, by no means. There were many interesting talks, there were just no breathtaking revelations. I myself am not an exception: I took the opportunity of having several loop-people in the audience to talk once more on the loop string, this time focussing on spontaneous breaking of diffeomorphism invariance.

By now, the PDFs are online and in a few days you will also find video footage. To get an idea what people discussed, the organizers had the idea to assemble tag clouds from the slides, some are above.

Let me mention a few presentations and speakers nevertheless. Steve Carlip talked about the notion of space-time being two dimensional at very short distances in several unrelated approaches. Related was a nice presentation of Silke Weinfurter on her papers with Visser on the scalar mode not decoupling in Horava gravity. That talk was probably on the most recent and hottest results and I had the impression that many other approaches still have to digest the lesson that it is non-trivial to modify gravity and still not throw out the baby with the bath tub.

Hermann Nicolai presented his work (together with Meissner) on a classically scale invariant version of the standard model in which the only dimensionful coupling (the Higgs squared term) arises from an anomaly. They claim that their model is compatible with the current data and would imply that LHC sees the Higgs and only the Higgs. Daniel Litim gave a nice overview over the asymptotic safety scenario for gravity. Bergshoeff and Skenderis talked about models related to 3d topologically massive gravity and Jose Figueroa-O-Farrill presented a summary of algebraic structures relevant for M2 theories.

Mavromatos discussed possible observations of time delays in gamma ray bursts and implications for bounding modifications of dispersion relations in quantum gravity. Steve Giddings talked about locality and unitarity in connection with black hole information loss and Catherine Meuseburger explained how in 3d gravity observers can make geometrical measurements with light rays to find the gauge invariant information on in which Ricci-flat world they are living.

I was surprised how many people still work on non-commutative geometry (in the various forms). The Moyal-plane, however, seems to be out of fashion (not so much because of UV-IR-mixing which I think is the main reason to be careful but many people seem to think they can work around that but are worried about unitarity on the other hand). Kappa-Minkowski is a space many people care about and Dopplicher explained why we live in quantum space-time. The general attitude seemed to be (surprisingly) that Lorentz-breaking in those theories is not an issue. However, Piacitelli, showed a calculation that should have been done quite a while ago: People say that although Lorentz invariance is broken that is not a problem since there is a twisted co-product version that preserves at least some related quantum symmetry. Piacitelly now spelled out what that means in everyday's terms: When you do a boost or rotation, twisting the co-product is equivalent to treating theta as a tensor and rotating that as well. Great, that explains why the formalism shows that rotational symmetry is preserved while the physics clearly says that a tensor background field singles out preferred directions. I had for a long time the suspicion that this is what is behind this Hopf-algebra approach but could never motivate myself enough to understand that in detail so I could confirm it.

In addition, there were many talks from loop-related people (also on spin foams, BF-type theories etc) about which I would like to mention just one: Modesto applied the reasoning found in the loop approach to cosmology (I would like to say more about this in a future posting) to a spherically symmetric space-time (i.e. what is Schwarzschild in the classical theory). What he finds is indeed Schwarzschild at large distances but the discretization inherent in that approach produces a solution that has a T-duality like R <--> l_p^2/R symmetry.

A great opportunity for meetings of this style with people coming from different approaches are always extended discussion sessions. Once more, those were a great plus (although not as controversial as a few years back in Bad Honnef), there were two, one on quantum gravity and one on non-commutative geometry.

There, once more, people complained that it is hard to do this kind of physics without new experimental input. Of course to a large degree, this is true. But to me it seems that also misses an important point: By no means, everything goes! At least you should be able to make sure you are really talking about gravity in the sense that in not so extreme regimes you recover well known physics (Newton's law for example). Above, I mentioned Horava gravity apparently failing that criterion and it seems many other approaches are not even there to be tested in that respect.

We often say, we work on strings because it is the only game in town. On that meeting you could have a rather different impression: It seemed more like everybody was playing more or less their on game and many didn't even know the name of their game. Another example of such a trivial non-trivial test is what your theory says about playing snooker: The kinetics of billard balls tests tensor products of Poincare representations of objects with trans-planckian momenta and energies. If your approach predicts weird stuff because it does not allow for trans-planckian energies my interpretation would be that you face hard times phenomenologically, even if your model agrees with CMB polarizations.

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## 9 comments:

Having not payed much attention to some of the more esoteric research directions in recent years, could you give us an impression about any progress people might have made?

Are the foundational problems plaguing say, LQG more or less the same as they were this time ten years ago or have they gotten some of them under control?

Did you hear Renate Loll's talk and what do you think about their approach?

Renate Loll's (and coworkers) approach is quite interesting and they have obtained impressive results. As always, her talk was very enjoyable, but I could not say that I remember hearing anything I did not hear before (same as for most of the other talks).

Their approach shares the problem of controlling the large scale, classical limit of the theory. In particular, as they treat time and space directions differently in their measure (which is a feature rather than a bug of their approach) it is not guaranteed that they obtain a Lorentz invariant theory in the continuum limit. They could share the fate of Horava gravity in having one of the many IR fixed points without that symmetry and thus be in conflict with classical gravity.

In a discussion Renate agreed with this assessment but pointed out that it is non-trivial to formulate this question in concrete form in their formalism to be tested for example numerically.

Re Anonymous1: I don't know which foundational problems you refer to in particular.

My impression is rather that many problems are avoided by not anymore working in old school loop quantum gravity any more but with all kinds of more or less related other approaches (BF, spin foams, loop cosmology etc).

Robert, it seems to me that you are less critical now of 'alternative' theories than in previous years.

Your own talk did not end with 'and therefore lqg quantization is wrong', but seems to leave it up to the audience to decide if alternative quantization of 2d gravity makes sense.

Is my impression correct?

How did you get that impression? I thought my critical attitude was expressed in this post.

And what did you expect? I would never say a thing like "your approach is wrong", not in public nor in a personal conversation. I would say things like "I am skeptic" or "there might be an issue with..." but I think everybody should be able to form their own opinion. Not all string theory bloggers have the same apodictic style.

In the case of the LQG string, I think my statement is clear: Restricting to invariant states is not physically justified as demonstrated by the Fock space Gupta-Bleuler style quantization of the string. This sheds serious doubts on the inevitability of discontinuities that I would say plague the loop approach and for example lead to ad hoc choices in the discretization of the analogue of the Hubble equation in the loop inspired approach to cosmology.

BTW I very much prefer to talk and discuss with people that do have a name.

I think NC geometry is a misnomer.

NC geometry defines a NC *-algebra rather than a spacetime geometry.

The spacetime geometry should be derived from the underlying NC *-algebra in the sense of emergent spacetime (or gravity). This is also true for a NC field theory.

Indeed the NC field theory can (or should) be regarded as a theory of gravity. (A recent review about this issue is, e.g., here. http://homepage.univie.ac.at/

harold.steinacker/Bayrischzell2009/Talks/yang.pdf) A basic motto about emergent spacetime is that an algebra defines a geometry. From this approach, it is important to notice that a NC gauge theory belongs to the (NC *-) algebra side and a spacetime geometry will be derived from the NC algebra defined by gauge fields. In particular, a noncommutativity parameter is a quantity defining a NC *-algebra,

NOT a tensor breaking Lorentz symmery. The Lorentz symmetry should be addressed from the geometry side which is derived from the algebra side. Then the Lorentz symmetry breaking due to a noncommutative spacetime is a fallacy since we are still stick to the conventional spacetime picture rather than an emergent spacetime. I think Piacitelli's interpretation is compatible with the emergent spacetime picture from NC geometry.

All videos are now available:

http://www.ift.uni.wroc.pl/~rdurka/planckscale/index-video.php

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